Organic light-emitting diodes (OLEDs) are thin film devices in which light is generated by applying a voltage over an organic film. In particular, OLEDs are increasingly used for signage lighting, such as advertising information and indication signs, but their use in flat panel displays, illumination and decorative lighting is also very promising.
In general, an OLED comprises a layer stack comprising an organic layer sandwiched between two electrodes, and optionally one or more organic charge injection or charge transport layers, arranged on a transparent substrate.
For commercialization of OLEDs, device efficiency is an important parameter. The total device efficiency is determined by the internal quantum efficiency and the external quantum efficiency (light outcoupling). In recent years many techniques have been proposed to improve the internal quantum efficiency, including for example doping of transport layers to increase charge recombination, and the use of triplet-emitter by which almost 100% internal quantum efficiency can be achieved. Also to improve the external quantum efficiency numerous techniques have been proposed, including particular stack designs for reducing external quantum efficiency roll-off of triplet-emitters based on reduced triplet-triplet annealing, and by choosing various layers with refractive indices and thicknesses such as to reduce reflection of the emitted light at the layer interfaces and thus reduce the considerable amount of light trapped due to total internal reflection within the organic layers, the electrode layers and/or the substrate layer. However, still as much as 60% of the generated light remains trapped in the organic layer, and about 20% remains trapped in the glass substrate.
In order to increase outcoupling of light trapped within the organic layer and/or the anode ITO layer (also referred to as light in “waveguide mode”), US 2010/018899 (Forrest et al.) proposed the use of a low refractive index grid embedded in the organic material. The outcoupling efficiency of such a device may be up to two to three times the efficiency of a standard OLED.
However, a problem with existing devices is that still, a relatively large portion of light remains trapped in the organic layers or the glass substrate due to total internal refection at layers interfaces, or is absorbed. Hence the need for improvement of the external quantum efficiency in OLEDs remains.